I would add that there are plenty of Z-Wave sockets on the market that are smart looking and work well, hence the lack of an ST branded one is less of an issue. I have found the ST ones unreliable and had the problem with it burning out due to its construction method (though they did replace it).
The SmartThings branded ones are Zigbee, for what that matters.
There are several recent threads in the form already on good pocket sockets for the UK, so rather than go and all of that again, hereās one that was recently updated: